Wireless communication devices generally transmit information by generating a radiofrequency carrier using a circuit such as an oscillator, and modulating information onto the carrier wave using amplitude modulation, frequency modulation, phase modulation, quadrature amplitude modulation (QAM) or other techniques including a combination of the aforementioned modulation types. Multiple such modulated signals may be combined to form more complex schemes such as orthogonal frequency division multiplexing (OFDM). The carrier is usually a sinusoidal voltage at a radio frequency; that is a frequency at which energy may be propagated in the form of an electromagnetic wave by connecting the sinusoidal voltage to an antenna. The modulation process modifies the amplitude, frequency, and/or phase of the carrier in a time varying manner to convey information. Examples of conventional wireless communication devices include analog communication systems such as analog AM and FM broadcast radio as well as digital communication systems such as the widely used Wi-Fi (e.g. IEEE 802.11) and Bluetooth data communication standards as well as digital television (e.g. DTV) and digital broadcast radio standards.
Generally, conventional wireless communication devices have radiofrequency carrier generation and the modulation processes carried out in a single device or installation of interconnected devices.
In contrast, backscatter devices generally refer to an alternative communication method where carrier generation and modulation are performed in separate devices. For example, a carrier frequency may be generated in a first device that emits an electromagnetic carrier wave. A second device carries out the modulation process by scattering or reflecting the carrier wave, thus affecting the amplitude, frequency, and/or phase of the carrier emitted by the first device. This can be achieved by modulated scattering; that is by selective reflection of the incident carrier wave by means of a modulator circuit. Backscatter devices, requiring a modulator which may be a simple as a transistor, may be quite simple and low power.
Backscatter communication is widely used in ultra-high frequency RFID systems. By using modulated backscatter to communicate, RFID tags are power efficient compared to alternative approaches using conventional wireless communication schemes. However, RFID tags require a specialized reader or receiver hardware to receive the backscattered signal. RFID readers, for example, are complex devices which include a transmitter circuit, which performs the carrier wave generation process, along with a receiver circuit, which receives the modulated backscatter signal and extracts the data transmitted by the RFID tag. This specialized hardware presents a cost and complexity burden to users of the RFID system, in that RFID readers must be purchased, installed, and maintained on a data communication network to take advantage of the RFID tags.